1. Field of the Invention
The present invention relates to a permanent magnet motor comprising a rotor including a rotor core made by stacking a plurality of steel plates and a number of permanent magnets serving as field magnets and inserted in magnet insertion holes formed in the rotor core so as to be peripherally aligned, and a washing machine provided with the permanent magnet motor as a drive source.
2. Description of the Related Art
The following motors have conventionally been suggested as motors of the above-described type. A first motor is a permanent magnet motor of the outer rotor type in which an annular rotor core has magnetic poles corresponding to permanent magnets respectively. Each magnetic pole has a magnetic pole face which is opposed to the stator and is an arc face with a center of rotation of the rotor (center of rotational axis) as an arc center. A second motor has as each magnetic pole face an arc face which has a center thereof on a magnetic pole axis passing the center of magnetic pole and is convex to the stator side. For example, JP-A-2004-254403 discloses a permanent magnet motor of this type.
In the motor of this type, the magnetic flux density distribution in the gap between the rotor and stator is shown as a sinusoidal wave. As a result, the motor of this type is superior in the torque development and in the oscillation and noise. On one hand, in the aforesaid first motor, the magnetic flux density distribution takes a trapezoidal shape. As a result, the first motor is low in the fundamental wave component effective for torque development and has a problem of large oscillation and noise.
On the other hand, each magnetic pole face has an arc face convex to the stator side. Accordingly, the magnetic flux density distribution is more sinusoidal in the second than in the first motor. However, the magnetic pole face is a single simple arc face convex to the stator side in the second motor. Accordingly, the magnetic flux of permanent magnets passes through the magnetic pole of the rotor core, concentrating on a single point of a peripherally central portion. The gap is the shortest between the point and the stator. As a result, the magnetic flux density distribution is approximate to a triangular wave, resulting in a problem of large oscillation and noise.